def evens(start,end):
    even = start +(start%2)
    while even<end:
        yield even 
        even += 2


def prefixes(s):
    if s:
        yield from prefixes(s[:-1])
        yield s

def substrings(s):
    if s:
        yield from prefixes(s)
        yield from substrings(s[1:])
        

